1. Field of the Invention
The present invention relates to optical amplifying devices for amplifying an input optical signal and, more specifically, to an optical amplifying device suitable for use in amplifying a burst optical signal.
2. Description of the Background Art
As well known, when intermittently-inputted optical signals (hereinafter referred to as burst optical signals) are amplified through a general optical fiber amplifier, for example, waveform degradation, called optical surges, occurs in the optical signals. Optical surges are now briefly described with reference to the accompanying drawings.
Optical surges are caused by transient response of optical amplifiers. How much the input optical signal is degraded in a waveform depends on the characteristics of the optical amplifier, such as a relaxation time constant. Waveform degradation also depends on the input optical signal itself. As the input light varies in power, the waveform becomes degraded.
FIG. 18a shows the waveform of an optical signal when the amount of data traffic is small and data is intermittently transmitted, such as a case where data packets are spaced long. If such burst optical signal as shown in FIG. 18a is provided to an optical amplifier, temporary periods during which no data is provided at all are observed, which are hereinafter referred to as a no-data period. If an optical signal is provided after a long no-data period, input light optical power varies. Therefore, as shown in FIG. 18b, the optical signal after amplification is instantaneously increased in level (optical surges), thereby causing degradation in the waveform.
Such waveform degradation in a transmission system makes it difficult for a receiving side to always optimally identify data. Thus, optical surges have to be suppressed. From this viewpoint, one optical amplifying device capable of carrying out optical amplification while suppressing optical surges is disclosed in Japanese Patent Laid-Open Publication No. 11-135862 (1999-135862). This conventional optical amplifying device (hereinafter referred to as conventional device) is described below with reference to the drawings.
As shown in FIG. 19, a conventional device 9000 is provided with an input optical signal of a wavelength λ1 as shown in FIG. 20a. The provided optical signal is branched into two by an optical brancher 910. One branched optical signal goes through an optical receiver 920, an inverting amplifier 940, and a light source 924, thereby being converted into an optical signal of a wavelength λd with its logic level inverted, as shown in FIG. 20b. Then, the converted optical signal is multiplexed with the other optical signal branched by the optical brancher 910. The optical signal after such multiplexing is constant in optical power, as shown in FIG. 20c.
The optical signal after multiplexing is amplified by an optical fiber amplifier 916. At this time, optical surges do not occur since the input light is constant in optical power. The amplified optical signal is provided to an optical filter 918, wherein the optical signal of the wavelength λ1 is passed through.
As such, according to the conventional device 9000, the input optical signal is superposed with a dummy optical signal having a different wavelength. Thus, the input light provided to the amplifier 916 can become temporarily constant in optical power. In this way, optical amplification can be carried while optical surges are suppressed.
As stated above, in the conventional device, the input optical signal is superposed with the dummy optical signal, and then provided to the amplifier. Therefore, the optical signal provided to the amplifier becomes larger in optical power on average than the input optical signal. In general, amplification gain of the amplifier varies according to the average optical power of the optical signal provided to the amplifier. The larger the optical power of the input light is, the less the amplification gain is. Therefore, in the conventional device, the amplification gain of the amplifier is disadvantageously reduced.
Moreover, the conventional device has to accurately detect data provided at a higher bit rate such as 10 gigabits/second for logic level inversion. Accordingly, the electrical load on the conventional device is increased. This increase leads to a degradation in device's performance and an increase in cost.
Also, a large number of components are required for the conventional device. Thus, the conventional device is complex in structure.